A bus is a portion of an electric power system from which individual feeder lines extend to individual users. Typically, the bus will include three-phase current values and IA, IB, IC and three phase voltage values VA, VB and VC. Many electric power protection applications include a bus total overcurrent system (BTOS), which measures all of the current entering and leaving a protected bus. The vector sum of the currents flowing through the circuit breakers connected to the bus is useful as an input to a time overcurrent element in a protective relay.
FIG. 1 shows a typical BTOS arrangement involving two feeder lines. With an electromechanical or other static relay, the three-phase currents on the breakers connected to the bus provide the BTOS value. For the circuit arrangement of FIG. 1, involving a source S referred to at 10, and feeder lines 12 and 14, the three-phase current at breakers 16 and 18 provide the BTOS current value for bus 20. The power system shown in FIG. 1 also includes a transformer 17 and a relay 19 for the transformer, associated with circuit breaker 22. To obtain the BTOS value for bus 20 in the circuit of FIG. 1, the polarity marks of current transformers 21 and 23 are arranged in common (hard wired), with the results connected to the relay 25. The relay arrangement of FIG. 1 will measure the total fault current for bus 20, but will measure zero current for out-of-section faults. The same is true for bus 24 and the arrangement of the protective relay, the circuit breakers and the current transformers associated therewith.
The traditional BTOS application has certain disadvantages; for instance, it is undesirable that the BTOS arrangement provides high-speed tripping in response to transformer low-voltage side faults. This is typically corrected by desensitizing the arrangement of FIG. 1. Tripping by the BTOS time overcurrent element will also typically delay the high speed tripping process, although this is frequently resolved by using a high set overcurrent element. Further, in those situations where the strength of the source 10 changes significantly over time, security can be compromised if there are no compensating elements. Hence, there are disadvantages and limitations of existing BTOS arrangements.